3. ABSOLUTE MAGNITUDE

3.1 Historical Galactic Supernovae

Among the historical Galactic supernovae, four candidate type Ia
events (and their remnants) are SN 185 (RCW 86), SN 1006 (PKS 1459-41), Tycho's supernova of 1572 (3C 10), and
Kepler's of 1604 (3C 358).
To determine their absolute magnitudes we need to know
distances, apparent magnitudes, and the interstellar
extinction. Distances can be inferred only from the remnants, and
apparent magnitudes only from contemporary records of the supernova
brightness. The interstellar extinction can be estimated either from
the remnants, from the extinction of field stars in the directions of
the remnants, or from contemporary descriptions of the supernova colors.

Strom (1988)
attempted to put the distances to the four remnants on
a self-consistent absolute scale. The known ages and measured angular
diameters of the remnants, together with the assumption that they are
in the adiabatic phase of their evolution, were first used to
determine relative distances to 10% accuracy, and then the scale was
fixed to within 20% by considering various determinations of the
absolute distances to individual remnants. In
Table 3 we list Strom's
estimates of the distances and their uncertainties. Distance
determinations not considered by
Strom (1988)
tend to suggest that the
remnant distance scale may need to be increased, making the absolute
magnitude brighter than in Table 3.
Westerlund (1969)
determined a distance of 2.5 kpc for the OB association near RCW 86 (but no
distance estimates for SN 185 have any basis if, as suggested by
Huang &
Moriarty-Schieven (1987),
RCW 86 is not the remnant of SN 185]. From
a new proper-motion study of PKS 1459-41Long et al
(1988) derive a
most probable distance range for SN 1006 of 2.1-2.7 kpc,
Fesen et al (1989)
find a range of 1.5-3.3 kpc on the basis of the background
Schweizer-Middleditch star, and
Smith et al (1991)
obtain 1.4-2.8 kpc
from a comparison of proper motions and the shock velocity from the
H
emission line, all to be compared to 1.4 kpc in
Table 3.
Smith et al (1991)
also obtain a range of 1.5-3.1 kpc for Tycho. From a bow shock
model Bandiera (1987)
estimates 4.5 ± 1.0 kpc for Kepler, but
Blair et al (1991)
estimate 2.9 ± 0.4 kpc from the shock emission method.
Van den Bergh &
Tammann (1991)
note that the surprisingly high local
supernova frequency that is implied by the historical events could be
reduced by increasing the remnant distance scale, again making the
absolute magnitudes in Table 3, and their mean,
brighter.

For the extinction of SN 185 we use AV = 1.8 ± 0.3.
Leibowitz & Danziger
(1983)
derived AV = 1.7 from the Balmer decrement in the RCW
86 remnant and
Westerlund (1969)
found AV = 2.0 for the OB
association. For SN 1006 we use AV = 0.3 ± 0.3 on
the basis of AV = 0.46 from the Balmer decrement in
the remnant
(Lasker 1981)
and AV = 0.28 for the total Galactic extinction in
that direction
(Burstein & Heiles
1978).
The extinction of SN 1572 and SN 1604 can be estimated
from their colors, but the result depends on whether they are assumed
to have been intrinsically blue like types Ia and II-linear (II-L),
with an intrinsic B - V
0.0 at maximum light, or redder like type Ib
and Ic. The SN 1572 and SN 1604 light-curve shapes are consistent with
all three types
(Doggett & Branch
1985).
For the intrinsically blue
case, we use AV 2.0 ± 0.3 for SN 1572
[de Vaucouleurs (1985)
derived AV
= 1.95 ± 0.1] and AV = 3.6 ± 0.5 for SN 1604
[Pskovskii (1978)
found AV = 3.81 ± 0.12,
Bandiera & van den
Bergh (1991)
prefer AV = 3.4 ± 0.9]. If SN
1572 and/or SN 1604 were intrinsically as red as SNe Ib/Ic, which
typically are 0.5 mag redder in B - V than SNe Ia
(L88,
Wheeler & Harkness
1990),
then the visual extinction derived from their colors
would be about 1.5 mag lower, as indicated in
Table 3. Both the high
and low values of extinction from the colors of SN 1604 are within the
range of independent estimates [e.g. AV = 2.2 by
van den Bergh &
Kamper (1977)
from foreground field stars, AV = 3.5 by
Danziger & Goss (1980),
and AV = 2.7 ± 0.3 by
Blair et al (1991)
from the Balmer
decrement in the remnant]. Field stars in the direction of SN 1572
suggest that, for a distance of 2.5 kpc, AV may be in
the range 1-3 mag
(Brodskaya &
Grigor'eva 1962),
which favors the higher extinction value for SN 1572.

Our tentative interpretation of the absolute magnitudes in
Table 3
is that SN 1572 was either a type Ib
(Green 1986,
Strom 1988),
a type II-L, or some other kind of event that is fainter than type Ia: and
that SN 185, SN 1006, and SN 1604 were type Ia. The only absolute
magnitude that can possibly be considered to be quantitatively
reliable is that of Kepler, M<V00 =
-19.7+0.6, and this relies on Kepler having been a highly extinguished type Ia.